3 UARTs, 1 I2C and SPI port. This board, unlike Teensy 2.0 and Teensy++ 2.0 works with 3.3V logic. Compared to Teensy 3.1 it has power regulator capable to provide 500mA and the board can be powered with 6V voltage.
You can program it with Arduino IDE or directly in C/C++ (this second option lets You unleash full ARM potential).

Small footprint (1.4" x 0.7") makes it ideal solution when You need small dev board and powerful processor.